Why was this one labelled "very hard" whereas the one on Sept 20th
was merely "Hard"?

I suspect that the grading is biassed towards or against some methods
of solution. I find that my methodology works very well with "very hard"
and their solutions emerge relatively easily but some of the medium and
hard puzzles take much more work - usually because fewer cells resolve
from my initial search for "mandatory pairs". That said, there are some
that resolve completely from "mandatory pairs" (plus simple elimination
by logic of course) without resorting to a solution grid.

Widening the subject, I find gradings in different puzzle sources to be
quite confusing. Some do not even have clarity (eg "challenging") as
to where they are in relation to other descriptors. It is a virtue of this
site that easy, medium and hard are well understood terms!

Why was this one labelled "very hard" whereas the one on Sept 20th
was merely "Hard"?

I suspect that the grading is biassed towards or against some methods
of solution. I find that my methodology works very well with "very hard"
and their solutions emerge relatively easily but some of the medium and
hard puzzles take much more work - usually because fewer cells resolve
from my initial search for "mandatory pairs". That said, there are some
that resolve completely from "mandatory pairs" (plus simple elimination
by logic of course) without resorting to a solution grid.

The grading method is subject to tinkering as it has yet to satisfy me entirely. Right now, any one of 4 or 5 different logic steps, even if only required once, makes a puzzle hard. The requirement to apply "hard" logic 3 or more times during solution bumps it up to very hard. Because the variety of hard logic is quite large, and because some brains spot certain patterns more easily than other brains, you may find some very hards OK and others close to impossible, but someone else may have the opposite experience.

(Incidentally, by your description above, I think all medium puzzles whould be resolvable without your solution grid -- that is simple elimination from definite numbers is enough. At anything more than beginner level, people solve medium puzzles using more advanced logic than is strictly required.)

> The grading method is subject to tinkering as it has yet to satisfy me
> entirely. Right now, any one of 4 or 5 different logic steps, even if only
> required once, makes a puzzle hard. The requirement to apply "hard"
> logic 3 or more times during solution bumps it up to very hard.

Thank you for this explanation.

> Incidentally, by your description above, I think all medium puzzles
> should be resolvable without your solution grid -- that is simple
> elimination from definite numbers is enough. At anything more than
> beginner level, people solve medium puzzles using more advanced
> logic than is strictly required.)

Great - a new challenge - to solve Medium puzzles without going into a
solution grid. Of course this may prove to be "hard" in practice!!